1. Field of the Invention
The present patent application relates to the field of pressure sealing devices in radar level gauging systems, and particularly to a pressure sealing device allowing for a reduction of waveguide diameter but with maintained cut-off frequency as well as a method for such sealing.
2. Description of the Related Art
Radar level gauges are commonly used today for measuring the level of the surface of a product kept in a container, such as a tank. These radar level gauges must be able to function under very different conditions. The products kept in the containers could be a lot of different products, such as petroleum refinery products, liquid gases and other chemical compounds. Thus, pressures and temperatures in the containers can have a wide range of values. Typical pressures can be 4-10 MPa and typical temperatures can be within the range of xe2x88x9240xc2x0 C.-+200xc2x0 C., but pressures and temperatures outside these values are also possible.
The level gauges typically comprise an antenna being fed by a waveguide, e.g. antenna using a horn fed by a circular waveguide. Other antennas in use are a parabolic antenna fed by a horn via a waveguide, a dielectric rod antenna or an array antenna fed by waveguide. Usually the waveguide and part of the horn is filled by a dielectric material an sealed by one or more O-rings. The dielectric material is arranged to provide a barrier for vapors or liquid that are in the interior of the container and to prevent the vapors from being discharged to the exterior. Since the containers often contains chemicals, the dielectric material used is preferably PTFE (Polytetraflourethylene) which simplifies the judgement of chemical compatibility. PPS (Polyphenylenesulphide) is another possible material for slightly higher temperatures, but having slightly less chemical resistance. For temperatures approaching 200xc2x0 C. the mechanical properties of PTFE are severely degraded, so the combination of high pressure and temperature require a good design of the pressure sealing enabling a good lateral fixing of the dielectric material. Still higher temperatures obviously needs other materials like quartz or ceramic materials, and by such materials the shape must be matched accordingly. Another type of material used as waveguide sealings is glass. In all those cases the waveguide is used to make the microwave transmission smooth and free of reflections in order to improve radar measuring accuracy but the sealing of the waveguide is an important part, not the least at high pressures. be matched accordingly. Another type of material used as waveguide sealings is glass. In all those cases the waveguide is used to make the microwave transmission smooth and free of reflections in order to improve radar measuring accuracy but the sealing of the waveguide is an important part, not the least at high pressures.
One prior art approach used so far is shown in FIG. 2. This prior art pressure sealing comprises a plug made of a dielectric material (e.g. PTFE), which is filling a waveguide feeding microwaves to a horn antenna made of a metallic material. The antenna comprises a flange arranged to mount the antenna on the roof of the container. The dielectric plug and the horn antenna are partly exposed to the container environment. Radar electronics (shown schematically in FIG. 1) is feeding the antenna through the waveguide, and is located outside the container. The dielectric plug is attached to a metallic short flange, which is secured to the horn antenna and flange by threads or similar elements. The plug is secured to the flange by a number of small circumferential ridges enabling a good lateral fixing of the plug. Furthermore, one or more sealing elements (not shown), such as O-rings, are arranged between the plug and the horn antenna.
However, this prior art approach decreases the diameter of the waveguide by the small circumferential ridges. The smaller diameter increases the cut-off frequency in a non-desired way, and the diameter outside the narrower section is already made as small as possible and the function of a level gauging system needs a rather wide bandwidth. Thus, there is little space for a narrower section to obtain a possibility to withstand typical mechanical forces caused by the container pressure. This type of sealing is adequate and widely used for container sealings at moderate pressures and temperatures.
Furthermore, different types of containers and gauging in different situations require use of different frequencies. Typical frequencies used in radar level gauges are 6, 10 or 26 GHz. Supposing that two versions of the electronics is available (such as 6 and 26 GHz) for use in different situations, it would be practical to have the same pressure sealing with a common type of antenna. With the same pressure sealing it would be possible to change frequency after the first installation and it would also simplify the logistics. Information on operation conditions may be scarce in advance so both the cases that the liquid is covered by foam, when 26 GHz will expect problems, and that the container contains more structures than expected, and a 6 GHz TX may expect problems, will occur. One practical arrangement in such level gauges is to split the gauge in two parts, one of which is a pressure sealing and the other an electronic unit in a separate enclosure. The pressure sealing, including the antenna, is mounted on a port in the container and will seal the container. The radar electronics enclosure including the waveguide feeding is mounted on top of the pressure sealing and may be mounted or removed without opening the container.
One problem with an arrangement where the same pressure sealing is used in a dual band system, is that the diameter of a waveguide for feeding the low frequency (6 GHz) must be considerably larger than the diameter of a waveguide feeding the high frequency (26 GHz). In case the waveguides are filled by PTFE, the diameter for the low frequency will be 24 -25 mm and the diameter for the high frequency 6 mm. Thus, in order to use the same pressure sealing, a waveguide having the larger diameter must be used. But when the high frequency propagates in a waveguide having a larger diameter than needed, a number of non-desired waveguide modes can propagate and great caution must be taken to avoid excitation of them.
Accordingly, it is an object of the present invention to provide an improved pressure sealing device allowing for an improved mechanical attachment of the dielectric waveguide filling material by a locally smaller diameter of the waveguide without increasing the cut-off frequency and resulting in a decreased mechanical strain and a reduced number of non-desired waveguide modes.
This object is achieved through providing a coaxially arranged conducting cylinder in the waveguide over a limited path along the waveguide.
Another object of the invention is to provide a method for improving the mechanical attachment of the dielectric waveguide filling material by a locally smaller diameter of the waveguide without increasing the cut-off frequency and which results in a decreased mechanical strain and a reduced number of non-desired waveguide modes.
This object is achieved through a method of providing a coaxially arranged conducting cylinder in the waveguide over a limited path along the waveguide.
A pressure sealing device and a method for decreasing the cut-off frequency in a pressure sealing device has been invented, where the diameter of the waveguide can be made much smaller than before due to a center conductor provided in the waveguide. The possibility to decrease the diameter is applied over a part along the waveguide (such as xcex/2) which enables the creation of shoulders or conical parts supporting a considerable mechanical force along the waveguide caused by a pressure in the container. The approach according to the present invention being advantageous in comparison to the previously discussed prior art approach, which increases the cut-off frequency in a non-desired way. The present invention eliminates this restriction of such a prior art approach through enabling a smaller diameter of the waveguide with maintained cut-off frequency.
Still other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.